1. Field of the Invention
The present invention relates to a gas measurement apparatus using Tunable Laser Absorption Spectroscopy (TLAS) and Wavelength Modulation Spectroscopy (WMS) and a method of setting a width of wavelength modulation in the gas measurement apparatus.
2. Description of the Related Art
Conventionally, a gas measurement apparatus using TLAS has been known. A laser used in TLAS oscillates a laser light whose intensity is determined by a driving current of the laser and temperatures. The gas measurement apparatus using TLAS measures parameters of a target gas by transmitting the laser light through the target gas and detecting the laser light transmitted through the target gas. For example, a concentration of a target component included in a gas can be measured. A ratio of an intensity of a laser light transmitted through a target gas relative to a laser light oscillated from a laser is defined as a transmittance T. T is expressed as T=e−εcl, where ε is a molar absorption coefficient, c is a concentration of a target component, and l is an optical path length. It is also known that a molar absorption coefficient ε depends on a temperature of a target gas, and that a shape of an absorption line depends on temperatures, pressures, and partial pressures of components coexisted in a target gas. A concentration of a target component can be determined by measuring a transmittance T if an optical path length l is known and it is known how a molar absorption coefficient ε and a shape of an absorption line depend on temperatures, pressures, and partial pressures of components coexisted in a target gas.
A wavelength modulation spectroscopy (WMS) can be used for measuring gas parameters using TLAS. In WMS, a laser light with modulated wavelength and/or intensity is oscillated by varying a driving current or a temperature of the laser. Then, the modulated laser light transmits through a target gas and the transmitted laser light is detected. A detection signal is then generated in accordance with the detected laser light. Further, a specific component of the detection signal is obtained by performing a phase-sensitive detection to the detection signal and a concentration of a target component is measured by using the detected specific component (For example, refer to Japanese Patent 2703835 and Japanese Patent 4467674).
In using WMS, an extent of a wavelength modulation can be expressed as a width of wavelength modulation. For an accurate measurement of gas parameters, this width needs to be appropriately set. The main reason for this is that a width of wavelength modulation is a factor that determines a resolution and a signal to noise ratio (expressed as a S/N ratio hereinafter) of an apparatus. A width of wavelength modulation is determined by an intensity of a modulation current included in the driving current. Therefore, the intensity of the modulation current needs to be set in order to set the width of wavelength modulation. Here, a variation rate of the wavelength to the driving current, namely a ratio of the variation of the wavelength of the laser light relative to the variation of the intensity of the driving current, is different between laser devices. Namely, since the intensity of an optimal modulation current is different between apparatuses, a setting of the modulation current needs to be performed in each apparatus. In general, the variation rate of the wavelength relative to the driving current changes over time by operating a laser. Therefore, in order for each apparatus to keep the width of wavelength modulation in an appropriate value, the modulation current needs to be adjusted in accordance with the operation of the apparatus.
A use of a wavelength measurement apparatus and a gas that has a known absorption line can be considered as means for setting a intensity of a modulation current. In using these means, a ratio of the variation of the wavelength of the laser light relative to the variation of the driving current is measured by measuring the wavelength of the laser light. Then, based on the measured ratio, the intensity of the modulation current is set to obtain a desired width of wavelength modulation. However, in using the above means, the following may be problems: (1) the gas appropriate for the wavelength measurement apparatus or the wavelength measurement needs to be prepared. In addition, the gas measurement apparatus and the wavelength measurement apparatus need to be moved in the case where the gas measurement apparatus has already been installed in the place where a target gas exists, because the gas measurement apparatus and the wavelength apparatus need to be installed to perform the wavelength measurement; and (2) a work efficiency is lowered and a time and an effort for setting the modulation current increases because the wavelength of the laser light needs to be measured.